1. Field of the Invention
The field of the invention is catalytic cracking of heavy hydrocarbon oils to lighter products in general, and unfavorable reactions occurring in a transfer line connecting the cracking reaction zone and a fractionator associated therewith, in particular.
2. Description of Related Art
Catalytic cracking is the backbone of many refineries. It converts heavy feeds into lighter products by catalytically cracking large molecules into smaller molecules. Catalytic cracking operates at low pressures, without hydrogen addition, in contrast to hydrocracking, which operates at high hydrogen partial pressures. Catalytic cracking is inherently safe as it operates with very little oil actually in inventory during the cracking process.
There are two main variants of the catalytic cracking process: moving bed and the far more popular and efficient fluidized bed process.
In moving bed cracking, the catalyst is in bead form. Feed contacts a moving bed of bead catalyst and is cracked into lighter products. The lighter products are removed from the reactor and charged via a transfer line to a distillation column, sometimes called the synthetic crude tower (Syntower) or the main column. In some moving bed units, the reactor effluent vapors were cooled in the transfer line just upstream of the main column, by injection of a recycle stream from the main column. The reactor effluent was cooled so that no superheated vapor would enter the column. Enough liquid was introduced into the transfer line just upstream of the main column to cool the effluent and produce a two phase mixture, which was charged to the base of the main column. Usually the liquid was injected by a single spray nozzle, which moved a lot of liquid into the transfer line, but did only a fair job of contacting the liquid spray with the hot vapor.
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking is endothermic, it consumes heat. The heat for cracking is supplied at first by the hot regenerated catalyst from the regenerator. Ultimately, it is the feed which supplies the heat needed to crack the feed. Some of the feed deposits as coke on the catalyst, and the burning of this coke generates heat in the regenerator, which is recycled to the reactor in the form of hot catalyst.
Catalytic cracking has undergone progressive development since the 40s. The trend of development of the fluid catalytic cracking (FCC) process has been to all riser cracking and use of zeolite catalysts.
Zeolite-containing catalysts having high activity and selectivity are now used in most FCC units. These catalysts work best when coke on the catalyst after regeneration is less e.g., less than 0.3 wt.
To regenerate FCC catalysts to these low residual carbon levels, and to burn CO completely to CO2 within the regenerator (to conserve heat and minimize air pollution) many FCC operators add a CO combustion promoter metal to the catalyst or to the regenerator.
U.S. Pat. Nos. 4,072,600 and 4,093,535, which are incorporated by reference, teach use of combustion-promoting metals such as Pt, Pd, Ir, Rh, Os, Ru and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.
Modern, zeolite based catalyst are so active that the heavy hydrocarbon feed can be cracked to lighter, more valuable products in much less time. Instead of dense bed cracking, with a hydrocarbon residence time of 20-60 seconds, much less contact time is needed. The desired conversion of feed can now be achieved in much less time, and more selectively, in a dilute phase, riser reactor.
Riser cracking is more selective than dense bed cracking. Refiners maximized riser cracking benefits, but in so doing induced, inadvertently, a significant amount of thermal cracking. Thermal cracking is not as selective as either riser cracking or dense bed cracking, and most refiners would deny doing any thermal cracking, while building and operating FCC units with all riser cracking which also did a significant amount of thermal cracking.
Thermal cracking was a by-product of upflow riser reactors, which discharged cracked products more than 100 feet up, and product fractionators which charged the hot vapors from the FCC unit to the bottom of the main column. The transfer lines to connect the FCC kept getting longer, and the material exiting the riser reactor kept getting hotter, and the combination caused thermal cracking. The trend to heavier feeds only made things worse. Higher temperatures were sought to crack the heavy feed, and the heavy feeds contained more highly aromatic material that wanted to thermally degrade to form coke or other undesired species.
The reasons for high risers in FCC, and for adding hot vapor to the bottom of the FCC main column will be briefly reviewed. After this, some other work on minimizing thermal cracking in riser cracking FCC units will be reviewed.
Risers are tall because of high vapor velocities and residence time. The FCC riser operates in dilute phase flow. There is better distribution of catalyst across the riser when vapor velocities are fairly high. Many FCC riser reactors now operate with vapor velocities on the order of 20-50 feet per second. To achieve enough residence time in the riser, the riser must be very tall. For a 2 second hydrocarbon residence time, the riser must be at least 100 feet long with a 50 fps vapor velocity. There usually must be addition space provided at the base of the riser reactor to add catalyst and more space for feed nozzles. The cracked vapor products exit the riser and enter a reactor vessel, at an elevation more than 100 feet in the air, for separation of spent catalyst from cracked products, usually in one or more stages of cyclone separation. The cracked products are eventually discharged, usually up, from the separation section, usually at an elevation well above the top of the riser, and charged to the base of the main column.
Hot vapors from the FCC unit are charged to the base of the main column for several reasons, but primarily so that the hot vapors may be used to heat the column. Another reason is that the hot vapors always contain some catalyst and catalyst fines, which are never completely removed in the FCC reactor, despite the use of multiple stages of cyclone separators. Adding the fines laden vapor to the bottom of the main column at least minimizes amount of fines that must circulate through the column. The fines are largely confined to the very base of the column. The lower trays or packing of the main column are designed to tolerate the fines by using sloping trays that permit fines to drain or be swept from a tray without clogging it.
The combination of high temperatures in the riser reactor, a tall riser reactor, and a bottom fed main column, give enough residence time to cause a significant amount of thermal cracking to occur.
As the process and catalyst improved, refiners attempted to use the process to upgrade a wider range of feedstocks, in particular, feedstocks that were heavier.
These heavier, dirtier feeds have placed a growing demand on the reactor and on the regenerator. Processing resids exacerbated existing problem areas in the riser reactor, namely feed vaporization, catalyst oil contact, accommodation of large molar volumes in the riser, and coking in the transfer line from the reactor to the main fractionator. Each of these problem areas will be briefly discussed.
Feed vaporization is a severe problem with heavy feeds such as resids. The heavy feeds are viscous and difficult to preheat in conventional preheaters. Most of the heating and vaporization of these feeds occurs in the base of the riser reactor, where feed contacts hot, regenerated catalyst. Because of the high boiling point, and high viscosity, of heavy feed, feed vaporization takes longer in the riser, and much of the riser length is wasted in simply vaporizing feed. Multiple feed nozzles, fog forming nozzles, etc., all help some, but most refiners simply add more atomizing steam. Use of large amounts of atomizing steam helps produce smaller sized feed droplets in the riser, and these smaller sized drops are more readily vaporized. With some resids, operation with 3-5 wt % steam, or even more, approaching in some instances 5-10 wt % of the resid feed, is needed to get adequate atomization of resid. All this steam helps vaporize the feed, but wastes energy because the steam is heated and later condensed. It also adds a lot of moles of material to the riser. The volume of steam approaches that of the volume of the vaporized resid in the base of the riser. This means that up to half of the riser volume is devoted to steaming (and deactivating) the catalyst, rather than cracking the feed.
In many FCC units better feed vaporization is achieved by using a higher temperature in the base of the riser reactor, and quenching the middle of the riser or the riser outlet.
Catalyst/oil contact is concerned with how efficiently the vaporized feed contacts catalyst in the riser. If feed vaporization and initial contacting of catalyst and oil is efficient, then catalyst/oil contact will tend to be efficient in the rest of the riser as well. High vapor velocities, and more turbulent flow, promote better contact of catalyst and oil in the riser. High superficial vapor velocities in the riser mean that longer risers are required to achieve the residence time needed to attain a given conversion of heavy feed to lighter components.
Large molar volumes are sometimes a problem when processing resids. This is because the heavy feeds, with an extremely high molecular weight, occupy little volume when first vaporized, but rapidly crack to produce a large molar expansion. Large amounts of vaporization steam add to the volume of material that must be processed in the riser, and addition of quench material to the riser, or to the riser outlet, all increase the volume of material that must be handled by the main column. More volume does not usually translate into reduced residence time in the transfer line connecting the cracked vapor outlet near the top of the FCC riser to the base of the main column. This is because refiners usually limit the vapor velocity in large vapor lines to about 120 to about 150 feet per second. Vapor velocities below this are used for several reasons, but primarily to control erosion and limit pressure drop. Erosion is a problem because of the presence of catalyst fines. Pressure drop is a problem, because it takes a lot of energy to transfer large volumes of material through a large pressure drop. High pressure drops in this transfer line, the line to the main column, would also increase the FCC reactor pressure, which is undesirable from a yield standpoint, and decrease the main column pressure which increases the load on the wet gas compressor associated with the main column.
Coking in the transfer lines connecting the FCC reactor vapor outlet with the main column refers to coke formation in this transfer line. FCC operators have long known that "dead spaces" in a line could lead to coke formation. Coke formation is a frequently encountered problem in the "dome" or large weldcap which forms the top of the vessel housing the riser reactor cyclones. If oil at high temperature is allowed to remain stagnant for a long time, it will slowly form coke. For this reason refiners have routinely added a small amount of "dome steam", typically 500 #/hr, to prevent formation of coke in the dome of an FCC unit. Coking in the transfer line is somewhat related, in that coke will form in stagnant or dead areas of the transfer line. Coke will also form if there are cool spots in the transfer line. The cool spots allow some of the heaviest material in the reactor effluent vapor to condense. These heavy materials, some of which may be entrained asphaltenic materials, will form coke if allowed to remain for a long time in the transfer line. Thus refiners have tried to insulate the transfer line to the main column, not only to prevent heat loss to the atmosphere, but also to prevent coking in this line. The problem of coke formation gets more severe with either an increase in reactor/transfer line temperatures, or with a decrease in feed quality so that it contains more heavier materials.
Although great strides have been taken to improve many parts of the FCC process, such as better regenerators, better catalyst strippers, and better catalysts, the process has not been able to realize its full potential, especially with heavy feedstocks including non-distillable materials.
These trends, to high temperatures and high vapor velocity in the riser, and tall risers, all improved the cracking process and provided better yields of cracked products. These trends allowed FCC units to process significantly heavier feeds. These trends also caused unselective thermal cracking of the valuable cracked products, and increased the amount of energy needed to move cracked products from the reactor to the main column.
We examined the work that others had done, and realized that it was time for a new approach. We wanted the benefits of short residence time riser cracking, without the unselective thermal cracking, coke formation in transfer lines, and excessive energy consumption associated with the conventional way of recovering cracked products from a FCC riser reactor vapors.
We wanted to be able to modify existing units to eliminate transfer line coking or transfer line thermal reactions, without completely rebuilding the unit. We needed to cool the reactor effluent vapor, but not waste the heat contained in the effluent vapor stream. We also wanted a system that would be reliable, could operate for years, be fail safe so that if it broke the unit could continue to operate, and that would not promote coking.
Some work was done on cooling FCC transfer lines, for different reasons. Moyer U.S. Pat. No. 3,338,821 was an FCC case, with quench just downstream of the FCC reactor. A primary reason for quenching was to reduce vapor velocity in the transfer line so that the high velocity stream of vapor and entrained FCC catalyst wouldn't wear a hole in the transfer line. Moyer was concerned that "the unavoidably entrained catalyst . . . at a tremendously high velocity . . . is undesirable because the finely divided particles of catalyst will erode the . . . conduit to a degree that frequent repairs are necessary." Col 1. lines 48-58. Moyer quenched to reduce vapor velocities in the transfer line. This also quenched the transfer line. At the relatively lower temperatures, and with the distillable feeds used, coke formation in the transfer line was not a problem. With higher reactor temperature feeds, and feeds (and products) containing more resid, the approach shown could cause transfer line coking. Wetting the hot wall of the transfer line with quench liquid would give the liquid enough residence time to form "coke" .
We believed that the problems of transfer line reactions could be solved with quench injection in the transfer line, provided a way could be devised to prevent wetting the transfer line at the point of quench injection. We wanted to retain the low pressure drop, and relatively low cost, associated with in-line, preferably co-current quench, but wanted to avoid forming a liquid film on any portion of the transfer line which was hot enough to cause coking.